This invention relates to an exhaust manifold of a multi-cylinder internal combustion engine.
In a multi-cylinder internal combustion engine, a construction that a plurality of cylinders are grouped in two groups in which exhaust strokes of cylinders are not successive and exhaust pipes communicating with the cylinders gather according to the respective groups is adopted in order to avoid an exhaust interference.
An exhaust manifold of such construction is made of cast iron generally, but in order to reduce the weight, there has been proposed an exhaust manifold constructed by plate metal members laid face to face with each other.
As an example thereof, an exhaust manifold described in Japanese Patent Publication No. Sho 63-26252 is shown in FIG. 12, which is an exploded view of the exhaust manifold 01. It is constituted by assembling three plate metal members, that is an upper side half body 02, a partition body 03 and a lower side half body 04, face to face with each other.
The upper side half body 02 is formed with four half walls 02a, 02b, 02c, 02d which swell out upward in shape of about semicircular cylinders parallel with each other and a confluence half wall 02e where inner two half walls 02b, 02c join to each other. The outer two half walls 02a, 02d merely project from the inner half walls 02b 02c outward and interiors of the outer half walls 02a, 02d do not communicate with the interior of the confluence half wall 02e.
On the one hand, the lower side half body 04 is formed with four half walls 04a, 04b, 04c, 04d which swell out downward in shape of about semicircular cylinders parallel with each other and a confluence half wall 04e where the outer half walls 04a, 04d join to each other. The inner two half walls 04b, 04c are merely positioned between the outer two half walls 04a, 04d and interiors of the inner half walls 04b, 04c do not communicate with the interior of the confluence half wall 04e.
The partition body 03 is formed with four half walls 03a, 03b, 03c, 03d which are semicircular cylindrical and parallel with each other and a confluence partition plate 03e of a flat plate shape. The inner two half walls 03b, 03c swell out downward, and the outer two half walls 03a, 03d swell out upward.
When the upper side half body 02, the partition body 03 and the lower side half body 04 are put together face to face, the inner half walls 02b, 02c of the upper half body 02 and the inner half walls 03b, 03c of the partition body 03 join together to form two inner exhaust pipes, the confluence half wall 02e and the confluence partition wall 03e form an inner side confluence section where the inner exhaust pipes join, the outer two half walls 04a, 04d of the lower side half body 04 and the outer two half walls 03a, 03d of the partition body 03 join together to form two outer exhaust pipes, and the confluence half wall 04e and the confluence partition plate 03e form an outer side confluence section where the outer exhaust pipes join.
The above-mentioned exhaust manifold 01 is relatively light in weight because it is constructed by plate metal members laid face to face with each other, but the individual exhaust pipe has a doubled half wall portion on a single wall formed by the half walls joined together.
Since sufficient strength and rigidity are ensured by the single wall, the doubled half wall portion is unnecessarily thick only to increase the weight uselessly. An exhaust manifold having appropriate wall thickness as a whole can not be formed, therefore, temperature of the exhaust immediately after starting is hardly transmitted to an exhaust cleaning catalyst apparatus (catalyst converter) so that activation of the catalyst is delayed and the exhaust cleaning at an early stage is difficult.
The present invention has been accomplished in view of the foregoing and an object of the invention is to provide an exhaust manifold made of plate metal in which sufficient strength and rigidity can be ensured, lightening of the weight is intended by making walls thin as a whole, and early activation of the catalyst is easy.